The classical concept of the arachidonic and/or linoleic acid cascade comprises the liberation of free polyenoic fatty acids from membrane phospholipids and subsequent oxygenation via the cyclooxygenase or lipoxygenase pathways forming biologically active compounds such as prostaglandins, thromboxane, and leukotrienes. Alternatively, 15- lipoxygenases can catalyze the direct oxygenation of esterified fatty acids within biomembranes. The generation of oxidized esterified lipids has been implicated as mediating a variety of physiologcial and pathological events. In Syrian hamster embryo(SHE) fibroblasts, the mitogenic polypeptide epidermal growth factor(EGF) stimulates the lipoxygenase oxidation of phospholipids. By reverse and straight phase-HPLC, chiral analysis, UV, and GC-MS techniques, we have identified the primary membrane product as 13(S)-hydroxy-linoleic acid. We characterized this compound as a potent enhancer of EGF-dependent DNA synthesis in SHE cells. We have also observed that the "leukocyte-type" but not the "platelet-type" 12-lipoxygenase can oxygenate complex ester lipids. Future investigations will analyze the specificity and selectivity of these enzymatic reactions and characterize the class of molecular species of phospholipid substrates. We will also examine oxygenation of cellular esters in relation to the functional role of lipoxygenases in modulating EGF-dependent fibroblast mitogenesis. These studies should expand current consideration of how mono-hydroxy fatty acids exert their biological actions and reveal possible mechanisms of interaction between oxygenated esterified lipids with transmembrane signal transduction pathways.